Two types of prior train approach detectors include those which respond to the presence of a train or other vehicle on a track without being sensitive to the speed at which the train is approaching the tie points of the detector to the track and, of course, those of the type which do respond to the train approach speed. The former type may simply include a section of track that is electrically insulated from adjacent portions of the track and a detector which detects electrical coupling of the rails of the insulated section through the wheels and axle, for example, of a train present thereon. The latter type have included so-called predictors, which upon detecting an approaching train at a particular location on a track predict the time of arrival at a protected area, i.e. the island, and movement detectors, which monitor the approaching train speed and distance from the island and, for example, drop a crossing gate prior to arrival of the train at a grade crossing in the island.
A variety of problems have been encountered with prior train approach detectors. For example, the track ballast condition, i.e. the lumped impedance seen by the detector, may vary widely with daily and seasonal weather changes often creating imbalances in the track signal monitored by the detector and, therefore, requiring frequent attention of a service person to adjust signal levels. Another problem experienced in the past has been distortion of the track signal by third harmonic radiation and entry into the detector of spurious signals. Moreover, the installing and servicing of prior train approach detectors have been relatively complicated, time consuming and, accordingly, expensive. Other specialized problems also have been encountered with the various prior train approach detectors.